The present invention relates to improved water gas shift catalysts and methods of their use for lowering the undesirable methane formation that can accompany water gas shift reaction processes, and in particular to lowering methane production in high temperature water gas shift reaction processes in a gas stream comprising hydrogen, steam and carbon monoxide. The catalysts and methods of the invention are useful, for example, in inhibiting methane production in a water gas shift reaction used to produce a hydrogen gas stream supplied to a fuel cell, particularly to proton exchange membrane (PEM) fuel cells.
Fuel cells directly convert chemical energy into electricity thereby eliminating the mechanical process steps that limit thermodynamic efficiency, and have been proposed as a power source for many applications. The fuel cell can be 2 to 3 times as efficient as the internal combustion engine with little, if any, emission of primary pollutants such as carbon monoxide, hydrocarbons and nitric oxides. Fuel cell-powered vehicles which reform hydrocarbons to power the fuel cells generate less carbon dioxide. (green house gas) and have enhanced fuel efficiency.
Fuel cells, including PEM fuel cells [also called solid polymer electrolyte or (SPE) fuel cells], as known in the art, generate electrical power in a chemical reaction between a reducing agent (hydrogen) and an oxidizing agent (oxygen) which are fed to the fuel cells. A PEM fuel cell comprises an anode and a cathode separated by a membrane which is usually an ion exchange resin membrane. The anode and cathode electrodes are typically constructed from finely divided carbon particles and proton conductive resin intermingled with the catalytic and carbon particles. In typical PEM fuel cell operation, hydrogen gas is electrolytically oxidized to hydrogen ions at the anode composed of platinum catalytic particles deposited on a conductive carbon electrode. The protons pass through the ion exchange resin membrane, which can be a fluoropolymer of sulfonic acid called a proton exchange membrane. H2O is produced when protons then combine with oxygen that has been electrolytically reduced at the cathode. The electrons flow through an external circuit in this process to do work, for example creating an electrical potential across the electrodes. Examples of membrane electrode assemblies and fuel cells are described in U.S. Pat. No. 5,272,017.
Fuel cells require both oxygen and a source of hydrogen to function. The oxygen can be readily obtained in pure form (i.e., O2) or from the air. However, hydrogen gas is not present in sufficient quantities in the air for fuel cell applications. The low volumetric energy density of isolated hydrogen gas compared to conventional hydrocarbon fuels makes the direct supply of hydrogen gas to fuel cells impractical for most applications because a very large volume of hydrogen gas would be required to provide an equivalent amount of energy stored in a much smaller volume of conventional hydrocarbon fuels such as natural gas, alcohol, oil or gasoline. Accordingly, the conversion of known hydrocarbon based fuel stocks to hydrogen gas is an attractive source of hydrogen for fuel cells and other applications.
Removal of impurities such as sulfur from the starting materials and lowering the concentration of oxidative products generated in the conversion process, such carbon monoxide, are major challenges in hydrogen production. Fuel cells are generally incapacitated by the presence of even low concentrations of CO, which poisons that catalyst at the anode. Despite development of more CO-tolerant Pt/Ru anodes, fuel cells are still susceptible to compromised function, for example when used with hydrogen sources with a CO concentration above 5 ppm.
The production of hydrogen gas from natural hydrocarbon sources is widely practiced in the chemical industry, for example in the production of ammonia and alcohol. A variety of reaction steps employing different carefully designed catalysts are used in the industrial production of hydrogen. A series of several reaction steps is typically required to reduce CO concentrations to below required levels, for example below 5 ppm. Many of these reaction steps require high pressures (for example, in excess of 1,000 psig), high reaction temperatures (for example, in excess of 800 deg. C.) and use self-heating pyrophoric catalysts. The scale and weight of machinery required to safely carry out such processes is too large for many fuel cell applications, such as automobile or residential applications. Furthermore, while the hazards presented by such reaction conditions can be effectively managed in an industrial production setting, similar hazards present unacceptable levels of risk for most fuel cell applications.
The water gas shift (WGS) reaction is a well known catalytic reaction which is used, among other things, to generate hydrogen by chemical reaction of CO with water vapor (H2O) according to the following stoichiometry:
CO+H2Oxe2x86x92CO2+H2
wherein the reaction requires a catalyst. Typical catalysts employed in this reaction are based on combinations of iron oxide with chromia at high temperatures (about 350 deg. C.) or mixtures of copper and zinc materials at lower temperatures (about 200 deg. C.).
When used at temperatures above about 300 degrees C., water gas shift reaction catalysts also cause the formation of methane (CH4) by catalyzing the reaction of CO or CO2 with hydrogen according to the reaction stoichiometries:
CO+3 H2xe2x86x92CH4+H2O
CO2+4 H2xe2x86x92CH4+2 H2O
The production of methane during the water gas shift reaction, also known as xe2x80x9cmethanationxe2x80x9d, is a side reaction that consumes hydrogen gas in an exothermic reaction. Thus, for applications where the water gas shift reaction is used to produce hydrogen gas and reduce CO concentration, the methanation reaction is a major disadvantage related primarily to precious metal containing water gas shift reaction catalysts. Methanation can reduce the hydrogen yield from the water gas shift reaction by consuming hydrogen to form methane, and increase the temperature of the catalyst thereby lowering the efficiency hydrogen production.
What is needed is a water gas shift reaction catalyst that inhibits or eliminates the methanation side reaction, that can be integrated with existing catalytic systems without significantly reducing the activity of commercially available catalysts and without significantly increasing the cost of catalyst synthesis and production. The present invention overcomes these deficiencies in the prior art by providing an improved precious metal water-gas shift reaction catalyst and methods for the use thereof.
In one embodiment, the invention relates to a process for carrying out the water gas shift reaction employing a methane production suppressing water gas shift reaction catalyst. The methane production suppressing water gas shift reaction catalyst comprises a methane production suppressing effective amount of a basic metal oxide. The basic metal oxide can be one or more of MgO, CaO, SrO, BaO, or ZnO. In one preferred embodiment of the process the basic metal oxide is zinc oxide calculated as ZnO.
In another embodiment the process employs a methane production suppressing water gas shift reaction catalyst that also has a support and a catalytic agent. In one embodiment the support is activated alumina. Preferably, the support has a BET effective surface area of at least 10 m2/g. In one preferred embodiment, the process employs a methane production suppressing water gas shift reaction catalyst that also has a promoter. The promoter can be one or more of CeO2, Nd2O3, Pr2O3, TiO2, Fe2O3, NiO, MnO2, or Co2O3. Preferably, the promoter is ceria calculated as CeO2.
In another preferred embodiment, the catalytic agent has one or more of Rh, Pd, or Pt.
In one aspect of the invention the process also employs a monolith, wherein the methane production suppressing water gas shift reaction catalyst is deposited on the monolith as a washcoat composition. In one preferred embodiment the basic metal oxide is ZnO. In another preferred embodiment the catalytic agent is Pt. In another preferred embodiment the support is alumina. In another preferred embodiment the catalytic agent is Pt and the support is alumina.
In another aspect of the invention the process also has the steps of: providing an input gas stream; contacting the input gas stream with the methane production suppressing water gas shift reaction catalyst; and catalyzing the water gas shift reaction with the methane production suppressing water gas shift reaction catalyst.
In one embodiment of this process, the methane production suppressing water gas shift reaction catalyst has: (i) alumina support particles; (ii) zinc oxide calculated as ZnO; and (iii) Pt. The input gas stream includes: (i) between about 1% by volume and about 10% by volume CO, (ii) at least 30% by volume hydrogen, and (iii) at least 15% by volume H2O. The input gas stream is characterized by a space velocity greater than 500 hrxe2x88x921 particle VHSV.
In another embodiment of the process, the methane production suppressing water gas shift reaction catalyst has: (i) a zinc oxide support particles calculated as ZnO, (ii) ceria calculated as CeO2 and (iii) Pt. The input gas stream includes: (i) between about 1% by volume and about 10% by volume CO, (ii) at least 30% by volume hydrogen, and (iii) at least 15% by volume H2O. The input gas stream is characterized by a space velocity greater than about 500 hrxe2x88x921 particle VHSV.
In another embodiment of the process, the methane production suppressing water gas shift reaction catalyst has: (i) an alumina support, (ii) zinc oxide calculated as ZnO and (iii) Pt. The methane production suppressing water gas shift catalyst is in the form of a washcoat composition deposited on a monolith. The input gas stream includes: (i) between about 1% by volume and about 10% by volume CO, (ii) at least 30% by volume hydrogen, and (iii) at least 15% by volume H2O. The input gas stream is characterized by a space velocity greater than 2,000 hrxe2x88x921 monolith VHSV.
In another aspect, the invention relates to a methane production suppressing water gas shift reaction catalyst having a methane production suppressing effective amount of a basic metal oxide and a catalytically effective amount of a catalytic agent. The catalytic agent is one or more of Rh, Pd, or Pt. In one preferred embodiment the basic metal oxide of the methane production suppressing water gas shift reaction catalyst is zinc oxide calculated as ZnO. In another preferred embodiment, the catalytic agent is Pt.
In another aspect, the methane production suppressing water gas shift reaction catalyst has: an alumina support impregnated with: (i) a catalytically effective amount of a catalytic agent and (ii) a basic metal oxide. The catalytic agent is one or more of Rh, Pd, or Pt. In one preferred embodiment the basic metal oxide of the methane production suppressing water gas shift reaction catalyst is zinc oxide calculated as ZnO. In another preferred embodiment, the methane production suppressing water gas shift reaction catalyst is in the form of a washcoat composition deposited on a monolith. In another preferred embodiment, the catalytic agent of the methane production suppressing water gas shift reaction catalyst is Pt.
In another aspect, the methane production suppressing water gas shift reaction catalyst has a support combined with a catalytically effective amount of a catalytic agent The support consists essentially of zinc oxide calculated as ZnO. The catalytic agent is selected from one or more of Rh, Pd, or Pt. In one preferred embodiment, the support of the methane production suppressing water gas shift reaction catalyst is further combined with a promoter. Preferably this promoter is ceria calculated as CeO2, and the catalytic agent is Pt.